If a gene encoding a protein whose third amino acid is tyrosine is affected by a single nucleotide substitution in that tyrosine codon, how many of the other 19 amino acids could possibly end up replacing tyrosine as the third amino acid?

6 different amino acids (no including Stop codons)

Explanation:

First of all, it is necessary to know that tyrosine is an amino acid coded by two different codons: UAU and UAC.

If there's a single nucleotide substitution, we need to redefine the new possible codons:

For UAU:

- Substitution in the first nucleotide:

* CAU: His

* GAU: Asp

* AAU: Asn

- Substitution in the second nucleotide:

* UCU: Ser

* UGU: Cys

* UUU: Phe

- Substitution in the third nucleotide:

* UAG: Stop

* UAC: Tyr

* UAA: Stop

For UAC:

- Substitution in the first nucleotide:

* CAC: His

* GAC: Asp

* AAC: Asn

- Substitution in the second nucleotide:

* UUC: Phe

* UGC: Cys

* UCC: Ser

- Substitution in the third nucleotide:

* UAG: Stop

* UAA: Stop

* UAC: Tyr

Possible amino acids replacing tyrosine (His, Asp, Asn, Phe, Cys, Ser)

c is having 4 valence electrons and is bonded to three hydrogens and one sulphur atom, therefore, the number of bond pair is equal to four. as all the valence electrons are used in forming the bonds so, there is no lone pair left on carbon.

s is having 6 valence electrons and is bonded to one other sulfur atom and one carbon atom, therefore, the number of bond pair is equal to 2. as only two valence electrons are used in forming the bonds so, four electrons are left. therefore, two lone pairs are present on s.